1. Field of the Invention
This invention relates to magnetic resonance imaging and, more particularly, to a method for producing an image of an object from a gradient echo pulse sequence produced in response to a back-to back pulse sequence. This invention further relates to a method for producing a selective component suppressed, three dimensional image of an object.
2. Description of Related Art
Magnetism results from the motion of electric charges such as electrons. Electrons can produce a magnetic field either by motion along a path or by virtue of their intrinsic spin. The particles that comprise the atomic nucleus, collectively called nucleons, also have spin and magnetic moment. Because both individual nucleons and nuclei have a charge distribution, rotation or spin of this charge produces a magnetic dipole whose value is called a magnetic moment. The numeric value of the magnetic moment determines the energies of the different orientations of a nucleus in an external magnetic field. The proton is positively charged and has a relatively large magnetic moment. Although neutral, the neutron also has a net magnetic moment. A neutron's magnetic moment is about two-thirds of the value of the proton's and points against the axis of spin. When in the nucleus, like nucleons align with their spins pointing against each other. This phenomena is called "pairing" and is favored because it leads to a lower nuclear energy state. Therefore, only the unpaired, odd proton or neutron, or both, contribute their magnetic moment to the nucleus. As a consequence, only nuclei with odd numbers of protons or neutrons, or both, have a magnetic moment. The magnetic properties of nuclei become important when they are placed in external magnetic fields as the nuclei will have a tendency to align with the external field.
Resonance occurs when an amount of energy equal to the difference of energy associated with the transition between states is absorbed or released. In the case of a magnetic moment of a nucleus, transitions between parallel or "up" and anti-parallel or "down" states can occur if the correct amount of energy is absorbed or released. Because the interaction is with a magnetic element, the necessary energy can be provided by a magnetic field. One way to obtain such a field is by utilizing electromagnetic radiations. To induce resonance, the frequency f of the electromagnetic radiation must be proportional to the local magnetic field H.sub.L. The particular proportionality constant which will induce resonance varies depending on the particular nucleus involved. The relationship between frequency and field is given by: EQU f=(gamma)H.sub.L /2(pi) (1)
where (gamma) is the magnetogyric ratio of the nucleus.
When the nuclei, originally in equilibrium with the field, are irradiated at the resonant frequency, the nuclei can adopt the anti-parallel state. When the return to equilibrium, if the field is unchanged, they will radiate emissions of the same frequency. If between excitation and radiation the field strength is changed, the nuclei will radiate a frequency corresponding to the new field value. This behavior of nuclei may be described by net magnetization vector N which characterizes the system by disregarding the state of each nucleus and considers only the net collective effect. In a magnetic field, the magnetization vector points along the field. The length of the magnetization vector is proportional to the number of nuclei in the sample and to the field strength and is inversely proportional to temperature. The length and direction of this vector characterizes the equilibrium magnetization of the sample; that is, the state that it will revert to after being disturbed if enough time is allowed to pass. This equilibrium magnetization is given by: EQU (mu).sup.2 H/kT (2)
where:
(mu) is the nuclear magnetic moment; PA1 k is Boltmann's constant; and PA1 T is the absolute temperature. PA1 M.sub.T0 is the value of M.sub.T immediately after irradiation; and PA1 t is the lapse time.
This vector can be disturbed from equilibrium by the application of a second external magnetic field. If such a field is superimposed upon the first magnetic field, M will align with the new net field. As M moves to its new direction, energy stored in the nuclei of the sample is provided by the second field. When the superimposed field is removed, M returns to equilibrium and the nuclei release the stored energy to the environment, either as heat or RF energy. These two fields are called the transverse field and the longitudinal field, respectively. More specifically, the component of M that points along the main field is called the longitudinal magnetization (M.sub.L) and the orthogonal component is called the transverse magnetization (M.sub.T). If the transverse field is an RF field at the resonant frequency, M behaves as a top such that, as it deviates from the longitudinal axis, it precesses about it. If the main magnetic field is defined as being aligned along the z axis, then M.sub.T rotates in the x,y plane and M.sub.L is reduced from its equilibrium value. If M is rotated onto the x,y plane by a 90 degree RF pulse, M.sub.L is 0.
Immediately after an RF irradiation, M.sub.L begins to grow again towards its equilibrium value M. This growth is exponential with a time constant T1 such that: EQU M.sub.L =M[1-exp(-t T1)] (3)
where t is the time since irradiation.
During this process, M.sub.T decays exponentially with a time constant T2 such that: EQU M.sub.T =M.sub.T0 exp(-t T2) (4)
where
When a proton is aligned with the magnetic field, it gives off no signal. When a proton is perpendicular to the field, it gives off a maximum signal. The rate at which a proton realigns with the static field is called its "T1" or "T1 relaxation time". The T1 relaxation time is also called "spin-lattice" or "thermal relaxation time". The individual protons exchange fixed amounts of energy when they flip from the down to up alignment in the process of returning to equilibrium. This exchange can occur only at the resonant frequency. A molecule in the lattice surrounding the resonant nucleus appears as an oscillating electric magnetic field with frequency that depends on its thermal velocity and mean free path. Since both vary over a broad range for any one temperature, of the whole ensemble of molecules, only a small fraction provide the right oscillating fields. These then coupled with the nucleus and allow the relaxation to occur. As temperature and molecular composition changes so does the distribution of velocities and mean free paths, thus affecting T1.
When a group of protons precess in phase, the voxel gives off a maximum signal. When a group of protons precess out of phase, the voxel gives off no signal. The rate at which the protons de-phase is called its "T2" or "T2 relaxation time". The T2 relaxation time is also called the "spin-spin" or "transverse relaxation time". In a perfectly uniform magnetic field, all nuclei will resonate at exactly the same frequency, but if the field is even slightly inhomogeneous, nuclei resonate at slightly different frequencies. Although immediately after an RF irradiation, the nuclei are all in phase, they soon lose coherence and the signal that is observed, decays. Any such loss of coherence shortens T2. Thus, the effects due to inhomogeneities in the external field produce a rapid decay characterized by the relaxation time T2.
Magnetic resonance has become an established method for producing an image of the internal structure of an object. Such methods have numerous applications particularly in medical diagnostic techniques. For example, the examination and diagnosis of possible internal derangements of the knee is one such application of magnetic resonance imaging techniques. Most magnetic resonance techniques for knee imaging use a two-dimensional (or "2 D") acquisition with a spin-echo pulse sequence to provide T1, T2 and proton density weighted images of the knee in multiple planes, typically the sagittal (y-z) and coronal (x-z) planes. However, the selective excitation techniques used by conventional 2 D methods is limited in the ability to obtain thin slices by the gradient strength of the system. Furthermore, obtaining images in non-orthogonal planes is often advantageous for proper medical diagnosis. However, to obtain images in a non-orthogonal plane, the use of 2 gradients rather than a single gradient is required to obtain a slice. Finally, oblique plane imaging of an object requires a corrected procedure after obtaining each gradient echo to keep the slices passing through the object being imaged.
As a result of the shortcomings of 2 D methods, three dimensional (or "3 D") acquisitions of magnetic resonance data has been used to produce thin slice, high resolution images. See, for example, the publications to Harms and Muschler, "Three-Dimensional MR Imaging of the Knee Using Surface Coils", Journal of Computer Assisted Tomography; 10(5): 773-777 (1986) and Sherry et al., (Spinal MR Imaging: Multiplanar Representation from a Single High Resolution 3 D Acquisition", Journal of Computer Assisted Tomography; 11(5): 859-862 (1987); Robert L. Tyrrell, "Fast Three-dimensional MR Imaging of the Knee: Comparison with Arthroscopy", Radiology; 166: 865-872 (1988); Charles E. Spritzer, et al., "MR Imaging of the Knee: Preliminary Results with a 3DFT GRASS Pulse Sequence", American Journal of Roentology; 150: 597-603 (1987); Alan M. Haggar, et al., "Meniscal Abnormalities of the Knee: 3DFT Fast-Scan GRASS MR Imaging", American Journal of Roentology; 150: 1341-1344 (1988).
It has long been desired to suppress the imaging of fat and/or water when producing MR images in connection with the examination and diagnosis of abnormalities of the orbit, head and neck, bone marrow, liver, breast and soft tissue masses as well as the cervical spine, knees, ankles, elbows, shoulder, wrist, lower legs, hips, thighs, and pelvis. Fat suppression has been long desired for T1 weighted sequences because MR images of fat generally tends to be of a sufficiently high signal intensity that lesions in anatomical parts with large amounts of fat, or where fat and other soft tissues are intermixed may be obscured. For example, the female breast is approximately 89% fat. When an MR image of a breast is produced, the fat appears as a high intensity image. However, a breast tumor would also appear as high intensity images in an MR image. As a result, the identification of a tumor using an MR image is difficult. This difficult diagnostic situation can be made even worse with the use of paramagnetic contrast agents that increase the signal of enhancing lesions by T1 shortening. As a result, the high signal intensity from contrast enhancing lesions can be made invisible by surrounding high signal intensity fat.
A variety of methods are available for producing fat suppressed images. One such technique is generally referred to as chemical shift imaging. As chemical shift imaging utilizes four dimensional imaging, it is very time consuming and highly inefficient, particularly if one considers that images from only two lines, fat and water, are required to produce fat suppressed images. Fat or water can be selectively saturated or excited. Selective saturation requires an additional RF pulse that lengthens the TR and is less effective with steady state sequences. Selective excitation is difficult considering RF and magnetic field inhomogeneities inherent to many objects.
Spectral information can also be obtained by phase encoding based upon differences in the chemical shift evolution between fat and water. Phase methods require two excitations and post-processing to yield fat and water images from in-phase and out-of-phase images. Differences in the relaxation properties of fat and water are used in STIR sequences to achieve fat suppressed images.
Recently introduced dedicated image processing work stations for medical imaging have made real time image plane reformatting, maximum pixel ray tracing (angiography), and surface renderings a clinical reality. These image processing routines are best performed on high resolution, thin slice image data sets that are optimally obtained by 3 D acquisition methods. The selective excitation technique used by conventional 2 D methods is limited in the ability to obtain thin slices by the gradient strength of the system and cannot approach the practical limit of 3 D acquisitions and slice thinness. A gradient echo sequence with a short TR is usually chosen for a 3 D acquisition. Most of the previously described fat suppression schemes cannot be used in a very short TR, and do not provide adequate SNR for thin slice 3 D acquisitions or require additional projections at length in the scan time.
Short TI inversion recovery (or "STIR") techniques require a long TR which would make a high resolution 3 D acquisition prohibitively long. The fat signal in STIR is suppressed by shorting the signal at the known T1 point of fat. BO or RF inhomogeneity could result in an inadequate fat suppression and bright spots in the image that could result in incorrect image interpretation. Since fat has a much shorter T1 than most other soft tissues, the measurement of STIR signals is early in the longitudinal relaxation of most tissues. As a result, fat suppression technique is performed at the expense of a lower signal-to-noise (or "SNR") ratio. Since a high SNR is critical for thin slices and a short TR is required a reasonable scan time, STIR cannot be effectively used for 3 D acquisitions.
Phase difference methods, often referred to as the Dixon method, can be used to produce summed and opposed phase images. These methods require at least two excitations that would double the scan time of an already lengthy 3 D acquisition time. One of the major limitations of phase difference methods is the increased sensitivity to motion artifacts that result not only from doubling the number of projections but from the knee for two projections for the calculations. Since these projections are required at different times during the scan, any movement between projections result in motion artifacts that are compounded in the image calculations. These effects are particularly pronounced in 3 D acquisitions which tend to be more motion sensitive than conventional 2 D studies because of the larger number of projections needed for image reconstruction. Also, while phase difference methods could be used for 3 D imaging, the problems of longer scan time and motion sensitivity make these methods a less attractive choice. Finally, variations in the magnetic field strength across the object causes phase errors which cannot be corrected.
The chopper method uses opposed directional selective excitation gradients and requires at least two selected radio frequency pulses in the sequence. Single RF pulse graded echo sequences could not use this method. To achieve the needed shifted fat and water resonance for fat suppression using the sequence, a narrow bandwidth, which can only be achieved at the expense of longer TE and TR values, is required. The echo times and repetition times needed for the few sequence choices that could employ this method would be prohibitively long for a 3 D acquistion.
Chemical shift selective presaturation (or "Chem-SAT") requires an additional radio frequency pulse in the sequence to presaturate fat. The RF pulse is a narrow bandwidth that requires a long excitation time, thereby resulting in a longer TR. Chemical shift presaturation produces non-resonant transverse magnetization that decreases the fat suppression effects and can result in artifacts when short TR steady-state sequences are employed. The longer TR, diminished fat suppression, and associated artifacts made Chem-SAT a less attractive choice for fat suppression and 3 D acquisitions.
It is an object of this invention to produce a selective, component-suppressed, three dimensional image of an object.
It is yet another object of this invention to provide a high resolution, magnetic resonance image of an object for use with paramagnetic contrast agents used for diagnostic purposes.
It is still yet another object of this invention to produce a pulse sequence which produces chemical shift selection in a steady state sequence with an improvement in signal-to-noise ratio that is optimized for producing high resolution, thin slice magnetic resonance images.
Yet another object of this invention is to produce an MR image of an object with minimal artifacts.